I do think there's regional variation. In general, we don't have this system in the US, except for a few grocery stores that are US divisions of European companies. I've lived in a few different areas in the country, and in some areas it seems like very few people return their carts and just leave them wherever, while in other areas, people are more conscientious about it. I rarely see a stray cart in the grocery store lots where I live now, for example, but when I lived for a few months in another city, it seemed like everyone just left the cars where they wanted.

I think there's kind of a peer pressure thing going on, too - people start doing one or the other, and everyone else follows. "He didn't return, why should I?" vs "Oh, she returned her cart, I should, too."